To manufacture a high quality paper, a great variety of attrition mills are used to refine the cellulosic pulp, the double disc pulp refiners being most extensively utilized.
The key components of known disc refiners are two coaxially arranged discs, their opposed operating surfaces being used to refine material passing therebetween. Widely used are dowble disc refiners having one disc held stationary and called a stator, and its other disc rotatably mounted and called a rotor. Each of the discs, the stator and the rotor, is formed by two parts, namely a base and a refining plate secured thereto. In most cases the refining plate is shaped like a metallic disc having refining bars mounted thereon and spaced to form channels therebetween.
The most general requirements imposed upon the refining process of the cellulosic pulp used for manufacture of paper possessing high mechanical strength, are to produce pulp products consisting of flexible and plastic fibres with maximum active surfaces, and to minimize the possibility of cutting the fibres. Moreover, the treated fibres must be napped that can be achieved by multiple brushing action of the refining plate upon the fibres.
It is difficult, however, to meet the requirements mentioned above, when using the refiner disc having a metallic refining plate, since the latter produces a considerable cutting action upon the fibres. In this case, the fibres are cut short without brushing and splitting and fall to reach the desired flexibility and plasticity. Manufactured from such a fibre product, the paper has loosely bound fibres that decrease its mechanical strength. The metallic refining plates exert a particular adverse effect when refining short-fibred wood chips since short-cutting of such fibres substantially decreases the paper's mechanical strength.
Known in the art are the discs having a refining plate made from sintered metal. A microporous structure of sintered metal permits the fibres to be brushed more intensively. The manufacture of the refining plate from a sintered metal is a very time-consuming operation, and said discs are not of wide application. The most common disadvantage of the discs provided with metallic refining plates is a high noise level arising from continuous interactions between the metallic plates of the rotor and the stator during the refining process.
There are also known refiners provided with two discs, namely the rotor having a metallic refining plate and the ceramic stator having its operating surface made from abrasive material which has an improved capability of brushing the fibres.
Rotor to stator clearance being negligible, the discs are repeatedly brought into contact during the refining operation, which results in rapid wear of the metallic refining plate of the rotor due to its contact with the abrasive surface of the stator, thus generating a need for often renewing the worn parts.
Also known is a refining disc having a refining plate comprising sections made from metal and ceramics (of. USSR Inventor's Certificate No. 502,993), which comprises a base supporting a combined metalceramic refining plate. The refining plate consists of a central metallic ring provided with refining bars and a peripheral ring made from ceramic-based abrasive material.
Such a disc is of limited utility since it is difficult to manufacture a ceramic-based abrasive ring of large diameter. Moreover, the peripherally arranged ring made from ceramic-based abrasive material hinders the pulp passing, thus decreasing the refiner output with simultaneous increase in the expenditure of energy.